The present invention relates to passive sensors in general and to ultrasonic passive sensors in particular.
Passive sensors (for implanting into the human body or for mounting at some inaccessible location within a machine) are known in the art. These sensors are typically electromagnetic, providing an electromagnetic signal when activated.
The prior art sensor systems typically comprise a sensor, implanted into the machine, and an activating and detecting system. The sensor is typically an oscillating circuit whose vibration frequency changes in response to the physical variable to be measured. The oscillating circuit typically includes a capacitor and an inductor, one of which is built to vary in accordance with the physical variable being measured. As a result, the vibration frequency of the circuit is a function of the physical variable.
When the sensor is irradiated with electromagnetic energy from the activating system, some of the energy is absorbed by the oscillating circuit, depending on how close the incident frequency or frequencies are to the resonant frequency of the circuit (which, in turn, depends on the physical variable being measured). The change in the electromagnetic field due to the absorption of energy by the oscillating circuit is detected by the detecting system.
Electromagnetic sensors and systems are described in the U.S. Pat. No. 4,127,110 and in an article: Carter C. Collins, xe2x80x9cMiniature Passive Pressure Transensor for Implanting in the Eyexe2x80x9d, IEEE Transactions on Bio-Medical Engineering, Vol. BME-14, No. 2, April 1967.
Unfortunately, within living tissue, the passive sensor is detectable within a range of approximately 10 times the diameter of its antenna (part of the oscillating circuit). Furthermore, the sensor system is not operative within a conductive enclosure.
Methods, devices and systems, using ultrasonically activated passive sensors usable for sensing different physical parameters within a human body or in other environments and scientific and industrial applications, have been described. U.S. Pat. No. 5,619,997 to Kaplan discloses a passive sensor system using ultrasonic energy. An ultrasonic activation and detection system ultrasonically activates passive sensors which may be implanted in a body or disposed in any other environment. The activated passive sensors or parts thereof vibrate or resonate at a frequency which is a function of the value of the physical variable to be measured. The passive sensors thus absorb ultrasonic energy from the exciting ultrasonic beam mostly at the frequency of vibration (resonance frequency) of the sensor. The frequency (or frequency range) at which the passive sensor absorbs energy may be detected by a suitable detector and used to determine the value of the physical parameter.
Additionally, if the exciting ultrasonic beam is pulsed, the ultrasonic sensor may continue to vibrate after the excitation beam is turned off. The frequency of the ultrasonic radiation emitted by the activated passive sensor after turning the excitation beam off may be detected and used to determine the value of the physical parameter.
Since more than one physical variable may influence the vibration frequency of passive sensors, a correction may be needed in order to compensate for the effects of other physical parameters unrelated to the physical parameter which needs to be determined on the measured sensor vibration frequency. For example, if pressure is the physical parameter to be determined, changes in temperature may affect the vibration frequency of the sensor. U.S. Pat. Nos. 5,989,190 and 6,083,165 to Kaplan disclose compensated sensor pairs and methods for their use for compensating for the effects of unrelated different physical variables on the determined value of another physical variable which is being determined.
Alternative methods for constructing and using passive ultrasonic sensors for performing measurements of a physical parameters may further advance the possibilities of performing measurements of physical parameters inside living organisms and in closed systems in industrial applications.
There is therefore provided in accordance with an embodiment of the present invention a passive acoustic sensor for determining the value of a physical variable in a measurement region. The sensor includes a housing having two spaced apart substantially parallel and substantially flat acoustically reflecting surfaces. At least one of the acoustically reflecting surfaces is a surface on a movable member configured to be movable with respect to the housing, such that the distance between the acoustically reflecting surfaces varies as a function of the physical variable. The acoustically reflecting surfaces are configured such that when incident acoustic waves having a range of frequencies are directed at the sensor in a direction substantially orthogonal to the acoustically reflecting surfaces, a first portion of the incident waves is reflected from one of the acoustically reflecting surfaces to form a first reflected wave, and a second portion of the incident waves is reflected from the remaining acoustically reflecting surface to form a second reflected wave. The first reflected wave and the second reflected wave interfere to form a returning acoustic signal having at least one maximally attenuated frequency which is correlated with the value of the physical variable in the measurement region in which the sensor is disposed.
Furthermore, in accordance with an embodiment of the present invention, one or more of the physical parameters of the sensor is selected such that the intensity of the first reflected wave is equal or substantially similar to the intensity of the second reflected wave.
Furthermore, in accordance with an embodiment of the present invention, one of the acoustically reflecting surfaces is a static surface of one of the walls of the housing.
Furthermore, in accordance with an embodiment of the present invention, one of the acoustically reflecting surfaces is a static surface of a wall of the housing. The housing has an open recess therein. The movable member is sealingly attached within the recess to form a sealed chamber within the housing. The chamber has a pressure level therein. The two acoustically reflecting surfaces are exposed on the external surface of the sensor for contacting a fluid within the region of measurement.
Furthermore, in accordance with an embodiment of the present invention, one or more of the parameters selected from the acoustic impedance of at least one component of the sensor, the area of the first reflecting surface of the two acoustically reflecting surfaces, the area of the second reflecting surface of the two acoustically reflecting surfaces, and any combinations thereof is selected such that the intensity of the first reflected wave is equal or substantially similar to the intensity of the second reflected wave.
Furthermore, in accordance with an embodiment of the present invention, the at least one component of the sensor is selected from the movable membrane of a portion thereof, and the wall of the housing underlying the static surface or a portion thereof, and the combination thereof.
Furthermore, in accordance with an embodiment of the present invention, the housing has an opening therein and a back wall opposing the opening. At least a part of the surface of the back wall facing the opening is the second reflecting surface of the two acoustically reflecting surfaces. The movable member is sealingly attached to the opening to form a sealed chamber within the housing. At least a portion of the surface of the movable member outside of the sealed chamber is the first reflecting surface of the two acoustically reflecting surfaces. The chamber has a fluid therein. At least a first part of the chamber defined between the movable member and the second reflecting surface is filled with the fluid. The sealed chamber includes at least a second part thereof. The second part of the sealed chamber is at least partially filled with a gas or a mixture of gases.
Furthermore, in accordance with an embodiment of the present invention, one or more of the physical parameters selected from the acoustic impedance of at least one component of the sensor, the thickness of the movable member, the area of the first reflecting surface of the two acoustically reflecting surfaces, the area of the second reflecting surface of the two acoustically reflecting surfaces, the acoustic impedance of the fluid within the sealed chamber, the distance between the movable member and the second reflecting surface, and any combinations thereof are selected such that the intensity of the first reflected wave is approximately equal to the intensity of the second reflected wave.
Furthermore, in accordance with an embodiment of the present invention, the at least one component of the sensor is selected from the movable membrane of a portion thereof, the back wall or a portion thereof, and the combination thereof.
Furthermore, in accordance with an embodiment of the present invention, the physical variable is the osmotic pressure in a first solution disposed in the measurement region. The housing has a second solution sealed therein. At least one component of the sensor selected from one or more of the walls of the housing and the movable member includes a semi-permeable material. The semi-permeable material is in contact with the first solution on one side thereof and with the second solution on another side thereof. The first solution includes at least one solvent capable of passing through the semi-permeable material, and the second solution includes at least one solute which cannot pass through the semi-permeable material.
Furthermore, in accordance with an embodiment of the present invention, the housing of the sensor is a hollow housing having a hollow passage passing therein. The movable member includes a flat member movably attached within the passage to sealingly close the passage. One of the two acoustically reflecting surfaces is the surface of a portion of the walls of the housing substantially parallel to the acoustically reflecting surface of the movable member. The housing is configured to be sealingly mounted within a wall of a vessel containing a fluid such that the two acoustically reflecting surfaces are in contact with the fluid, and the physical variable is the pressure within the fluid.
There is also provided in accordance with another embodiment of the present invention a passive acoustic sensor for determining the value of a physical variable in a measurement region. The sensor includes a housing. The sensor also includes a first reflecting means formed in the housing or attached thereto. The first reflecting means has a first substantially flat acoustically reflecting surface for reflecting a first portion of incident acoustic waves directed perpendicular to the surface to form a first reflected wave. The sensor also includes a second acoustically reflecting means formed in the housing or attached thereto. The second reflecting means has a second substantially flat acoustically reflecting surface substantially parallel to the first surface for reflecting a second portion of the incident acoustic waves to form a second reflected wave. At least one of the first acoustically reflecting surface and second acoustically reflecting surface is a surface on a movable member configured to be movable with respect to the housing such that the distance between the first acoustically reflecting surface and the second acoustically reflecting surface varies as a function of the physical variable. The first and second acoustically reflecting means are configured such that the first reflected wave and the second reflected wave interfere to form a returning acoustic signal having at least one maximally attenuated frequency which is correlated with the value of the physical variable in the measurement region in which the sensor is disposed.
Furthermore, in accordance with an embodiment of the present invention, one or more of the physical parameters of the sensor is selected such that the intensity of the first reflected wave is substantially similar to the intensity of the second reflected wave to maximize the attenuation of the maximally attenuated frequency in the returning acoustic signal.
There is also provided in accordance with another embodiment of the present invention, a system for determining the value of a physical variable in a measurement region. The system includes at least one acoustic transducer configured for directing acoustic waves having a range of frequencies towards a passive acoustic sensor disposed in the measurement region. The system also includes at least one acoustic receiver configured for receiving acoustic waves reflected from the passive acoustic sensor to generate a received signal. The system also includes at least one passive acoustic sensor. The sensor includes a housing having at least two spaced apart substantially parallel and substantially flat acoustically reflecting surfaces. At least one of the acoustically reflecting surfaces is a surface on a movable member configured to be movable with respect to the housing such that the distance between the acoustically reflecting surfaces varies as a function of the physical variable. The system also includes a controller unit operatively coupled to the transducer(s) and to the receiver(s), for controlling the operation of the transducer(s) and of the receiver(s). The controller is configured for acquiring data representing the received signal, processing the data to determine the value of at least one maximal attenuation frequency within the range of frequencies, and determining the value of the physical variable from the value of the maximal attenuation frequency or frequencies.
Furthermore, in accordance with an embodiment of the present invention, at least one of the acoustic transducer(s) and the acoustic receiver(s) includes at least one piezoelectric device.
Furthermore, in accordance with an embodiment of the present invention, the acoustic transducer is a piezoelectric transducer, and the piezoelectric transducer is configured to operate as the acoustic receiver.
There is further provided, in accordance with another embodiment of the present invention, a method for determining a physical variable in a measurement region using a passive acoustic sensor. The method includes the step of disposing in the measurement region a passive acoustic sensor. The sensor includes a housing having at least two spaced apart substantially parallel and substantially flat acoustically reflecting surfaces. At least one of the acoustically reflecting surfaces is a surface on a movable member configured to be movable with respect to the housing such that the distance between the acoustically reflecting surfaces varies as a function of the physical variable. The method also includes the step of directing acoustic waves having a range of frequencies at the sensor such that a first portion of the waves is reflected from one of the acoustically reflecting surfaces to form a first reflected wave and a second portion of the incident waves is reflected from the remaining acoustically reflecting surface to form a second reflected wave. The first reflected wave and the second reflected wave interfere to form a returning acoustic signal. The method also includes the step of acquiring data representing the returning acoustic signal. The method also includes the step of processing the data to determine the value of at least one maximal attenuation frequency, and the step of determining the value of the physical variable from the value of the maximal attenuation frequency (or frequencies).
Furthermore, in accordance with an embodiment of the present invention, the physical variable is the pressure in the measurement region.
Furthermore, in accordance with an embodiment of the present invention, the acoustic waves comprise sonic waves and ultrasonic waves.
Furthermore, in accordance with an embodiment of the method of the present invention, the physical variable is the osmotic pressure in a first solution disposed in the measurement region. The housing has a second solution sealed therein. At least one component of the sensor selected from one or more of the walls of the housing and the movable member includes a semi-permeable material. The semi-permeable material is in contact with the first solution on one side thereof and with the second solution on another side thereof. The first solution includes at least one solvent capable of passing through the semi-permeable material and the second solution includes at least one solute which cannot pass through the semi-permeable material.
Furthermore, in accordance with an embodiment of the present invention, the step of processing includes performing frequency domain analysis of the data to obtain frequency domain data of the returning acoustic signal and determining least one maximal attenuation frequency from the frequency domain data.
Furthermore, in accordance with an embodiment of the present invention, the frequency domain analysis includes performing a Fourier transform on the data to obtain Fourier transform data representing the intensity of the returning acoustic signal as a function of frequency, and determining the maximal attenuation frequency or frequencies from the Fourier transform data.
Furthermore, in accordance with an embodiment of the present invention, the frequency domain analysis includes performing a wavelet transform on the data to obtain wavelet transform data, and determining the maximal attenuation frequency or frequencies from the wavelet transform data.
Furthermore, in accordance with an embodiment of the present invention, the step of determining includes determining the value of the physical variable from the value of the maximal attenuation frequency or frequencies using a look up table or other calibration data obtained by calibrating the sensor.
Furthermore, in accordance with an embodiment of the present invention, the acoustic waves directed at the sensor are selected from a continuous beam of acoustic waves, one or more pulses of acoustic waves, chirped acoustic waves spanning the range of frequencies, and a tone burst series spanning the range of frequencies.
Furthermore, in accordance with an embodiment of the present invention, the method further includes the step of processing the data to compensate for variations in the intensity of the acoustic waves directed at the sensor at different frequencies. The variations are introduced by the characteristics of the acoustic transducer used to produce the acoustic waves.
Furthermore, in accordance with an embodiment of the present invention, the step of directing includes directing the acoustic waves towards the sensor in a direction substantially perpendicular to the acoustically reflecting surfaces. There is further provided, in accordance with another embodiment of the present invention, a method for using a passive acoustic sensor including a housing having at least two spaced apart substantially parallel and substantially flat acoustically reflecting surfaces. At least one of the acoustically reflecting surfaces is a surface on a movable member configured to be movable with respect to the housing such that the distance between the acoustically reflecting surfaces varies as a function of the physical variable. The sensor is disposed in a measurement region. The method includes the step of directing acoustic waves having a range of frequencies at the passive sensor such that a first portion of the waves is reflected from one of the acoustically reflecting surfaces to form a first reflected wave and a second portion of the incident waves is reflected from the remaining acoustically reflecting surface to form a second reflected wave. The first reflected wave and the second reflected wave interfere to form a returning acoustic signal. The method also includes the step of acquiring data representing the returning acoustic signal. The method also includes the step of processing the data to determine the value of at least one maximal attenuation frequency within the range of frequencies.
Furthermore, in accordance with an embodiment of the present invention, the method further includes the step of determining the value of a physical variable in the measurement region from the value of at least one maximal attenuation frequency.
Furthermore, in accordance with an embodiment of the method of the present invention, the physical variable is the pressure in the measurement region.
Finally, in accordance with an embodiment of the method of the present invention, the physical variable is the osmotic pressure in a first solution disposed in the measurement region. The housing has a second solution sealed therein. At least one component of the sensor selected from one or more of the walls of the housing and the movable member includes a semi-permeable material. The semi-permeable material is in contact with the first solution on one side thereof and with the second solution on another side thereof. The first solution includes at least one solvent capable of passing through the semi-permeable material, and the second solution includes at least one solute which cannot pass through the semi-permeable material.